By way of general background, lithography is used to transfer a specific pattern onto a surface. Lithography can be used to transfer a variety of patterns including, for example, painting, printing, and the like. More recently, lithographic techniques have become widespread for use in “microfabrication”—a major (but non-limiting) example of which is the manufacture of integrated circuits such as computer chips or semiconductor wafers.
In a typical non-limiting microfabrication operation, lithography is used to define patterns for miniature electrical circuits. Lithography defines a pattern specifying the location of metal, insulators, doped regions, and other features of a circuit printed on a silicon wafer or other substrate. The resulting semiconductor circuit can perform any of a number of different functions. For example, an entire computer can be placed on a chip.
Improvements in lithography have been mainly responsible for the explosive growth of computers in particular and the semiconductor industry in general. The major improvements in lithography are mainly a result of a decrease in the minimum feature size (improvement in resolution). This improvement allows for an increase in the number of transistors on a single chip (and in the speed at which these transistors can operate). For example, the computer circuitry that would have filled an entire room in 1960's technology can now be placed on a silicon “die” the size of a thumbnail. A device the size of a wristwatch can contain more computing power than the largest computers of several decades ago.
One idea to improve lithography performance is to use a programmable mask to expose the substrate. Generally, a programmable mask is a large array of “pixels” that can be individually controlled to either be open (transmit light to the substrate) or be closed (not transmit light to the substrate). There have been several suggested mechanisms for making programmable masks. One is to use liquid crystals to rotate the polarization of light incident on a pixel. In this case only the rotated (or not rotated) polarization would be transmitted to the substrate, and the other polarization would be blocked. Another mechanism for making the pixels is to use mechanical mirrors that can move to either reflect light into or out of the optics of the lithography system. Yet another mechanism is to use electric fields to make semiconductor pixels either transparent or not transparent (pixels made using the semiconductors or liquid crystals can also be referred to as shutters). By individually controlling the shutters, any desired pattern can be easily produced and then easily changed to produce any other pattern. See for example commonly-assigned U.S. Pat. No. 6,291,110 to Cooper et al. entitled “Methods For Transferring A Two-Dimensional Programmable Exposure Pattern For Photolithography” incorporated herein by reference.
Using a programmable mask allows the lithography process to have a high throughput as in conventional parallel lithography since a large number of features are printed in each step. A non-exhaustive list of some of example and illustrative features and advantages provided by performing lithography using a programmable mask may be found in the above Cooper et al. U.S. Pat. No. 6,291,110.
While programmable masks have the potential to fundamentally improve modern photolithography, further improvements are possible and desirable to take better advantage of programmable lithographic techniques and to solve problems related to the use of programmable lithography. We have developed such improvements and enhancements in the following areas:                pattern decomposition methods and systems that control the shutter opening and closing, and movement of, a programmable mask to create a desired pattern;        methods and systems that use predetermined phase shifting material on exposure pixels to optimize the basic patterns to be exposed on a semiconductor wafer or other substrate;        programmable phase-shifting methods and systems that use and control programmable mask shutters to programmably control the phase of photons passing through the mask;        methods and systems that use apodization to tailor photon distribution at the resist.        
These various techniques can be used independently, together in any combination, and/or in combination with other techniques (e.g., photoresist exposure techniques such as disclosed in our commonly-assigned application Ser. No. 10/298,224 filed Nov. 18, 2002, now U.S. Pat. No. 6,879,376, based on provisional application No. 60/331,515 filed Nov. 19, 2001), to improve performance such as resolution of programmable photolithography.
For example, one issue that arises is the variability of feature placement and size. In conventional parallel lithography, the feature size and pitch are limited by the smallest achievable intensity profile. However, the features can be placed with an accuracy significantly greater than resolution, and can have an arbitrary size so long as it is larger than the minimum resolution. With a programmable mask, the shutters are spaced at regular intervals so it can be more difficult to place a feature with very high accuracy, or of arbitrary size. Using each shutter in a programmable mask to expose a portion of resist equal in size to the single shutter intensity profile may limit the minimum feature size to the size of the single shutter intensity profile.
One way of dealing with this is to have the exposure system do multiple exposures. In between each exposure, it is possible to move the mask a small amount relative to the wafer. A combination of multiple exposures and movement of the mask relative to the wafer may correct for defective pixels and allow one to choose the location of the feature.
Another way of dealing with feature size limitations due to pixel size and to diffraction limits is to use one (or more) darkfield exposure(s) in combination with programmable lithography, in such a way that the inherent limitations of the darkfield method (excessive space between features) is overcome. This is achieved by overlapping pixel images at the resist in such a way as to create dark regions which are closely spaced, as detailed below.
Yet another approach to improved resolution is to directly modify the single pixel intensity profile (the spatial distribution of energy at the resist due to a single shutter, or pixel) so as to improve the overall flexibility of the programmable lithography system. In certain circumstances it is advantageous to have a steep sided intensity profile, such as in the case where features are created using pixel overlap. In other circumstances it is advantageous to have a peaked intensity profile, such as in the case where feature widths are adjusted by adjusting the amount of time during which light is permitted to fall on the resist (timing control).
Another area in which programmable lithography can be improved is in the case where there is some amount of overlap (due to diffraction) between the light distribution of adjacent shutters. Depending on the pattern being exposed, this may be either desirable or undesirable. In the case where it is desirable, obviously we need do nothing. But in the case where overlap is undesirable, we can compensate for its effects by placing a phase shifting material on one or both shutters, so that the light passing through one shutter is phase-shifted, e.g. by 180 degrees, relative to the light passing through the other shutter. The light in the region of overlap then interferes destructively, reducing the total energy deposited in the overlap region. However, we cannot necessarily predict a priori whether or not we will need to phase-shift a particular set of shutters, since the same mask will be used to expose multiple patterns.
We can solve this problem of arbitrary feature size and placement by, for example, using multiple exposures with local control of the exposure timing, by the use of a phase shifted shape library, by use of pixel-by-pixel programmable phase shift, and by the use of apodization of the limiting aperture of the optical system.
For example, an illustrative method for decomposing a desired resist exposure pattern and using the decomposed pattern to perform programmable lithography involves expressing the desired resist exposure pattern in vector form, and expressing the relationship between the shutter energies and the resulting total energy delivered to the various regions of the resist as a matrix. The pseudo-inverse of the matrix is then calculated, and applied to the desired resist exposure pattern in vector form in order to generate a vector representing the shutter exposure energies. A mask is programmed using the generated shutter exposure energy vector. Electromagnetic energy is then passed through the programmed mask to expose a substrate having resist disposed thereon.
Another example non-limiting method for exposing a resist by use of programmable lithography involves the use of a library of shapes. These shapes can for example be created by the use of phase shifting or other means. The desired pattern of resist exposure is built up by successive exposures of the resist, with possible relative movement of the programmable mask and the wafer between exposures.
A material with changeable refractive index can be applied to the shutters of a programmable mask, or on a separate submask interposed between the programmable mask and the resist, such that the phase of the light from an individual shutter may be programmably changed so as to modify the intensity distribution of light impinging upon the resist.
A material can be applied to the limiting aperture of a lithography system, with specified refractive index and transparency such that the phase and amplitude of the light passing through the limiting aperture are modified so as to create features on the resist smaller than the features created by the system in the absence of said material.